1. Technical Field
The present invention relates to an inventory tracking system used in a shipping container storage yard. More particularly, the present invention relates to the association of inventory handling equipment (HE) used to move containers in a shipping container storage yard with the containers moved by the HE as located by the inventory tracking system.
2. Related Art
Over the recent decade, the number of shipping containers handled in container yards has increased dramatically. To improve the efficiency of container terminal material handling processes, inventory tracking systems have been developed to track and monitor what really takes place in the yard. Such an inventory tracking system can employ real-time positioning technology (such as Global Positioning System (GPS) and Real-time Locating System (RTLS)) and wireless communications to track locations of containers by actively tracking the movement and locations of container handling equipment (HEs) that pick up, move, and set down the containers. The inventory tracking system then records the tracking information to an inventory tracking database and interfaces with a Terminal Operating System (TOS) to update container locations whenever a HE picks up or sets down a container. Such inventory tracking systems are designed to improve the accuracy of the container yard inventory and thereby reduce lost containers, maximize TOS performance, and improve the efficiency of HEs.
HEs can be categorized into two broad types: (1) lift equipment—any type of HE that is capable of lifting a container and setting it down on the ground, on top of another container, or onto another HE for transportation, and (2) transport equipment—any type of HE that is capable of moving a container from one location to another but is not capable of lifting the container and setting it down. Typical lift equipment includes top picks (also referred to as top lifts or top loaders), side picks (also referred to as side lifts, side loaders or empty handlers), reach stackers, straddle carriers, rubber tired gantries (RTGs), rail mounted gantries (RMGs), and quay cranes. Typical transport equipment includes over-the-road (OTR) trucks and tractors (also referred to as yard tractor, tug, UTR (Universal Tractor Ross), jockey truck, hustler, yard hustler, etc.). Both OTR trucks and tractors can have an attached chassis (also referred to as street chassis, trailer, bomb cart, yard chassis, terminal trailer, and so on) which are capable of carrying one or more containers.
In this description, lift equipment is referred to as container handling equipment (CHE) and transport equipment is referred to as tractor-type HE or simply as tractors. And HE is used for both CHE and tractors.
Since inventory tracking systems track the containers by tracking pieces of HE, it is critical to ensure that the association between a container and the HE that moves it is correct. For example, assume a CHE or a unit of lift equipment picks up a container, Container C1 so labeled for purposes of discussion, from its location in a shipping yard and puts it onto the chassis of a tractor labeled Tractor1 for this discussion. The system then associates Container C1 with Tractor1 and identifies the location of Tractor1 as the location of Container C1 until the next transaction on Container C1.
Next consider a situation where the CHE sets down Container C1 onto the chassis of Tractor1, but there happens to be another tractor, Tractor2, right behind Tractor1 waiting for its turn to receive a container. Further assume that due to errors in the positioning sensors and therefore errors in the position estimates of the two tractors, the position estimate of Tractor2 appears to be closer to that of the CHE than the position estimate of Tractor1. The system identifies Tractor2 as the tractor that receives Container C1 from the CHE and associates Container C1 with Tractor2. As a result, the system assigns the location of Tractor2 as the location of Container C1, while in reality Container C1 is on top of Tractor1. Moreover, the error propagates since such an erroneous association leads to erroneous associations in subsequent transactions on Container C1. Thus, erroneous tracking and location information has been generated due to incorrect associations between containers and HEs.
To ensure correct association between containers and HEs, the inventory tracking system needs to correctly identify the HEs involved in each container inventory transaction. Typically, the inventory tracking system recognizes a container transaction based on information from sensors such as the twistlock sensors/switches. Twistlocks are mechanisms installed at the four corners of a CHE's spreader bar to secure containers in transit on the CHEs. Twistlock systems include electronic components (e.g., motors, control circuits, and sensors/switches) to enable remote operation of the mechanical components. Two types of sensors/switches are typically used to facilitate the operation of an electronic twistlock. For the convenience of description, these two types of sensors/switches are referred to herein as twistlock contact sensors and twistlock engagement switches. When a CHE picks up or sets down a container, these twistlock sensors/switches change their outputs or status indication. For example, before a CHE picks up a container, all four twistlocks at the four corners of the spreader bar of the CHE must be engaged to secure the container onto the spreader bar for lifting; as the twistlocks are engaged, their corresponding twistlock engagement switches will change their status from “disengaged” (or “unlocked”) to “engaged” (or “locked”) to indicate the engagement of the twistlock. Based on this change, the CHE can detect the occurrence of a container pickup operation and report to the inventory tracking system its own ID, the pickup operation, and the time the operation occurs. Similarly, when a CHE sets down a container, the twistlocks are disengaged to release the container from the spreader bar, which leads to a change from “engaged” (or “locked”) to “disengaged” (or “unlocked”) in the status of twistlock engagement switches. Thus, the inventory tracking system in this way obtains information regarding the operation (picking up or dropping off) and the CHE that performs the operation. The correct association between containers and HEs then hinges on the correct recognition of the tractors that receive or provide the containers.
To ensure correct association between containers and HEs, some prior art inventory tracking systems have transponders and readers installed on the HEs. In one prior art system, transponders are installed on tractors while readers are installed on CHEs. Whenever a CHE picks up a container from (or drops off a container onto) the chassis of a tractor, the readers on the CHE read the transponders on the tractor and identify the tractor based on the information (e.g., tractor ID) from the transponders. However, such transponder-reader-based mechanisms require line of sight or close proximity for readers to read transponders. Despite the fact that multiple readers/transponders are typically installed on each HE to maximize the possibility of line of sight/proximity (which results in a higher cost), readers often still fail to detect transponders on tractors. Moreover, the transponder-reader-based mechanisms are vulnerable to interference and require a relatively high level of maintenance.
Other prior art inventory tracking systems install bar codes on tractors and scanners (or cameras) on CHEs. Similar to transponder-reader-based mechanisms, the scanners (or cameras) on a CHE read the bar codes on the tractor and the tractor is identified based on the bar codes. The drawbacks of such mechanisms include the line-of-sight requirement, performance degradation due to bad weather (especially if cameras are used) as well as damage to or soiling of the bar code (making it unreadable). In addition, the location of the bar codes can also be a problem. For example, a bar code on the side of a tractor can be difficult for a crane, which is much taller than a tractor, to read. Also a bar code on top of a tractor is difficult for a top pick, which is not much higher than a tractor, to read. To solve this problem scanners or cameras can be installed on a top pick's spreader bar, which can be much higher than a tractor, but scanners or cameras installed on a spreader bar are prone to damage.
Some other inventory tracking systems simply employ higher-accuracy positioning sensors such as GPS sensors to improve the positioning accuracy; however, this solution leads to a (significantly) higher cost. Furthermore, the improvement in accuracy is still limited due to the harsh environment for the positioning sensors. For a GPS-based positioning system, multipath and long periods of GPS blockage are common in a canyon-like environment formed by stacked containers surrounding the tractors, which limit the achievable accuracy even for a higher-accuracy GPS (or DGPS).